The present invention relates to the calibration of a digital camera, and in particular to calibration using feedback from a display.
Calibration is an issue for all types of imaging systems. For example, in scanners, it is typical to include a card which the user feeds into the scanner, with a card having different patterns and colors for calibration of the scanner. U.S. Pat. No. 6,141,121 shows a scanner which prints a color chart, and then scans its. U.S. Pat. No. 5,884,118 prints an image and then automatically feeds it to the scanning path for calibration.
U.S. Pat. No. 5,803,570 shows a camera picking up images from a projector by being in the projection path. This image is then fed back to the image generator controlling the projector to improve image quality.
Other methods exist for calibrating a display, such as a CRT. U.S. Pat. No. 4,754,329 shows a method for calibrating an oscilloscope display by placing a video camera over the oscilloscope screen, and providing the image to a separate monitoring display. U.S. Pat. No. 5,216,504 shows a camera which is used to provide a feed-back signal for a display. The feedback signal is used to adjust the settings for the red, green and blue guns of a CRT display, along with other adjustments.
U.S. Pat. No. 5,606,365 shows correction data for a camera being available from the factory and delivered electronically over a network, such as the internet. The camera can provide raw data to the computer which is then corrected with the downloaded correction data to provide a corrected display from the camera.
U.S. Pat. No. 5,918,192 teaches a calibration system where a camera is pointed at a display, and the picture the camera sees on the display is fed back to the computer. The pixels on the display are illuminated individually, or in groups, with the camera picking up the image. From the camera response, the computer can identify problem pixels of the camera and store a correction factor. This procedure should be done using a very good display at a factory, to insure the corrections are for the camera, not the display. The corrections can be shipped with a disk accompanying the camera. The user can perform the same procedure with the user""s display, with this user procedure then correcting for variations in the pixels due to the user""s particular display. However, this requires precise positioning of the camera to pick up and identify each pixel.
The present invention provides a calibration system that is practical and doesn""t require precise positioning of the camera. All or a significant portion of the display is activated with the same value, such as with the same color value. The camera is pointed at the display, but doesn""t have to precisely pick up the whole display. The computer compares the color information received by the camera with the color information sent to the display, and will adjust camera parameters until the comparison results in a match.
The camera is calibrated using a sequence of display feedback operations in one embodiment. All or part of the display is captured by the camera, and correction factors are stored to correct the value for each parameter (color, offset, brightness, etc.). For example, the correction factor for different colors can be adjusted, so that if a color value of 128 is sent to the display, the same value of 128 is detected by the camera. The process is then repeated, to see if the corrected numbers now match what is expected. If not, a further fine-tuning is done until the color numbers output from the camera equal the numbers provided to the display.
The present invention overcomes the shortcomings of the prior art to provide a practical system. Not only does the user not have to precisely position the camera, jitter due to use hand movement is not a problem since this smoothes out the color information from the various pixels on the display, and actually improves the calibration. Instead of trying to calibrate each pixel in a display, the invention calibrates based on the value of the whole display, or at least a majority of it. In one embodiment, the different values received at the camera sensor are averaged, with positions off by more than a predetermined amount being discarded (for portions of the camera sensor not pointed at the display, for example).
In one embodiment, a cross-hair or other target is provided on the display so the computer can tell where the camera is pointed, and make appropriate adjustments. To insure that a sufficient number of sensing pixels on the camera are used in the calibration, the user can be instructed, by a message on the display, to move the camera closer, or to the left, etc., so that the field of view of the camera includes enough the display.
In one embodiment, two calibration tables are provided. The first calibration is done at the factory for all cameras of a particular model. The factory calibration table is established with correction factors for each of the parameters tested (color, brightness, etc.) and is stored in the driver software. A customer can then set up the particular camera that customer purchases with that customer""s own computer and monitor, and provide a customized calibration by pointing the camera at the display and going through the same steps. Any additional calibration required for that particular camera and that particular display is then stored in a customer calibration table, which is combined with the factory calibration values to provide the overall correction. In this manner, the factory calibration is maintained, and can be resorted to as a default in case the user wants to calibrate to a separate computer and display.
For a further understanding of the nature and advantages of the invention, reference should be made to the following description taken in conjunction with a couple of drawings.